Ed Note: After this issue of Industrial Aviation had gone to press, news of the B-29 raid was released. To give our readers additional technical information not available in the general press, the story, which had been prepared and cleared in advance, is presented here as an insert.
News of the electrifying Boeing B-29 Superfortress raid on Yawata and other Japanese war production centers by the 20th Air Force on June 9, broke silence on one of the more closely guarded secrets of the war. This raid had a special significance for your reporter for he was one of the news, magazine and radio men who were invited recently to Wichita to see and ride in the big plane which by now has without doubt made our enemy change his whole concept of war in the air. Make no mistake about this new, big bomber. Right now, it is the queen of the skies a remarkable airplane in more ways than one and an aerodynamic marvel with her new wing and tail and streamlining. Take that from one who has flown in her for 2 hr and who has marveled at her gyrations as she was so easily handled high in the sky by competent Lieut Col Ralph G Vaughan, the Army's resident representative at Boeing's Wichita plant. Even now one cannot say too much about Superfortress. There are many things about her our enemies would pay any cost to know. But you can be told that she handles easily remarkably so for a plane so big, and she is majestically stable, virtually noiseless and vibrationless in operation. You may know she is a monument to the wizardry of Edison for she is virtually an all-electric airplane and has about 150 electric motors of 49 different types. The only non-electrical actuated units are the big propellers and brakes.
You can also be told that she carries the greatest load of any other airplane in existence faster, farther and higher. While gross weight is about thrice that of the Flying Fortress, her tremendous power comes from four Wright R-3350 Double Row, 18-cylinder radial, air-cooled engines, each rated at 2200 hp for takeoff. They are being built by the designers, Wright Aeronautical Corp, and the Dodge Chicago plant of Chrysler. The Dodge plant, built specifically for the Superfortress is the largest such plant in the United States. Propellers are four-bladed Hamilton Standard Hydromatics with a diameter of 16' 7" the largest now available.
Built especially for Superfortress, reduction gears turn the propellers only 35/100ths times as fast as crankshaft rpm the lowest ratio ever used. This was necessary since, for aerodynamic reasons, it was essential to keep propeller tip speeds below the speed of sound and yet utilize maximum engine power. Although the propellers turn slower than those on contemporary planes, speed of the propeller tips, because of their diameter, compares with tip speeds on other planes.
An interesting fact is that the Army ordered production started before experimental models had been completed.
Fuselage is all-metal, semi-monocoque design, with an all-aluminum-alloy stressed skin. It is long and perfectly cylindrical in shape, having slender tapering wings, and a single fin and rudder closely resembling the dorsal fin of the Fortress and Stratoliner.
The elongated nose, which projects well ahead of the wings, is one of Superfortress' distinguishing features. Pilot, co-pilot and bombardier sit well forward in the spacious splendor of the big cockpit from which visibility is virtually unlimited due to the expansive use of transparent plastics. Behind them are the radio operator, navigator and flight engineer. As in other types, there is a tail gunner. Sleeping quarters are also provided.
Flush-riveted and butt-jointed externally throughout, this new sky queen is aerodynamically by far the cleanest large plane yet designed, incorporating unique engine nacelle design, enclosed turbosuperchargers, flush drains, vents and high streamlining. She is a fine production airplane, easier manufactured than most aircraft.
First concern of the design engineers was the development of a wing that would deliver the required performance. Several wings were developed which finally resulted in the Boeing "117" airfoil, a wing with remarkable qualities exceeding those of any previous design.
Because of refinements in drag characteristics, this wing offered greater range performance. The Army, uncertain of the wing because of its high loading, feared it would reduce ceiling and affect handling characteristics during takeoffs an landings. The result was the novel Boeing-designed flap which counteracted high wing loading. After wind tunnel tests, construction of three experimental models was ordered.
The "117" wing is of web-type construction as compared to the tubular-spar construction of the Fortress wing. A heavy duralumin chord was used for the web spar. When machined, this 255-lb chord is the largest extrusion used in production planes. Improved features of the wing include: decreased drag per lb of lift; increased thickness of interspar area permitting greater strength of the primary structure and greater volume for carrying gasoline; improved stall warning; more gradual stalling characteristics. An unusually slender wing, it has an aspect ratio of 11.5. Since a lower percentage of air loss occurs at the wing tips, this high aspect ratio results in high wing efficiency.
Except for the portion between the inboard nacelles and the fuselage, the trailing edge is approximately straight. The flap trailing edge is approximately straight. The flap trailing edge in this portion hooks downward thus decreasing aerodynamic interference between the wing and body, and minimizing tail buffeting during climb.
Her stability, both in direction and roll, is due to the 4½° dihedral, the 7° sweepback and the large vertical fin which suppresses tendencies to slip or skid. One of her desirable features is that any tendency to fall off on one wing during a stall is immediately converted into a bank and then into a glide, straight ahead.
To decrease takeoff distance and speed, the flaps are extended 25° at takeoff. They roll back and down, increasing the wing area and the downwash angle of the air leaving the wing. Total wing area is increased 19% when the flaps are extended.
Developing control surfaces for adequate control and maximum stability for a plane of this size presented many problems. It was thought that hydraulic boost of flight controls would be necessary. After extensive research, new control surfaces were developed and are so closely balanced, both aerodynamically and statically, that no power or boost is required to move the controls. Involving a better "pilot feel," this system was designed with less cost, less vulnerability and less weight than the boost system. Operation of the rudder actually requires less effort than that of the Fortress, no power or boost being necessary.
Ailerons are not only aerodynamically and statically balanced but also have combination servo and trim tabs to reduce control effort to a minimum, providing a greater rolling response than that which is found in previous large aircraft.
Although of different airfoil and construction, stabilizers and elevators are identical in planform and size with those used on present B-17s. Stalling of the stabilizer at a critical fight attitude is prevented by the modification of the stabilizer airfoil so that the leading edge is turned up, in this manner giving the effect of an inverted airfoil.
Design of the nacelle, which had to be large enough to accommodate the dual turbosuperchargers and intercoolers, involved a lengthy process of aerodynamic evaluation. Consideration also had to be given to provide adequate space in which to retract the landing gear. Clay models of nacelles were made for more than a year before the design was finally adopted since it was necessary to arrive at a compromise between the demands of the aerodynamics, and structural and mechanical design units of the engineering departments.
Aerodynamics engineers wanted a nacelle which would allow the plane all possible speed while the project engineers strove for one which would house all needed equipment.
The nacelle design finally adopted has proved practical for all purposes, and, for their size, more air is drawn through them than any others now being used.
Since Superfortress is a high altitude plane, the installation of turbosuperchargers was an important operation.
It was found that no single supercharger was large enough to supply the high-powered engines. A twin installation which placed two superchargers on each engine was therefore designed. Developed and proved through a long test program, this novel installation was a complete success.
Except for the bomb bays and part of the fuselage between waist and tail gun positions, the fuselage is supercharged for high altitudes. Early in the Superfortress program, exhaustive gunfire damage tests were conducted on pressurized B-29 fuselages. It was found that only the most extensive battle damage at extreme altitudes will have any effect on the crew. In the event of cabin pressure failure, however, each member of the crew is equipped with emergency oxygen supply.
Superfortress pressurized cabin is intended primarily for crew comfort. Though little attention had been given to the fatigue and physical hardship suffered by crew members of the original Fortresses and Liberators, those factors have contributed more to aircrew deficiencies than anything else. Crew quarters in Superfortress have been designed to overcome those handicaps.
At high altitudes, the tube also is pressurized, thus enabling crew members to move through the bomb bay area from one pressurized compartment of the airplane to the other. The cabin superchargers automatically turned on at 8000 ft; the maximum cabin "altitude" we reached was 8200 ft, while the airplane was at 20,000 ft.
One sizeable undertaking was the design of a tricycle landing gear. Because of the plane's weight, a dual nose wheel was essential. Heretofore, no one had ever designed a dual nose wheel for aircraft and in particular for one whose nose wheels were 3' in diameter. The design, needless to say, was finally accomplished successfully and provision made for retraction.
Another problem was retracting the main gear into the nacelles. Boeing engineers, striving for aerodynamic perfection, wished them to retract completely rather than have them protrude slightly. Squeezing the main wheels, 4' 8" in diameter with dual tires, into the nacelles was not an easy problem and in order to streamline this section completely, doors had to be installed to close behind the wheels.
A scaled-down model was constructed and installed on a Douglas A-20 Havoc at Wright Field for test purposes. Trials proved that even if one tire blew out the gear operated satisfactorily another successful engineering achievement. As an illustration of the plane's aerodynamic qualities the drag is doubled when the landing gear is down.
Electrically-sighted and -fired armament consists of power turrets with multiple gun installations. Guns in present models are 20-mm and .50-cal.
Five different plants, operated by three major companies, are involved in the production program. They are the Seattle, Renton, and Wichita plants of Boeing; Bell Aircraft's Marietta plant; and Glenn L Martin's new Omaha plant. Production of the Flying Fortress will be continued by Douglas and Lockheed.
Although Boeing faced a tremendous problem in placing the B-29 in quantity production before the prototype airplane had been flown, especially since many items which had not been proved in flight might be subject to drastic change, this possibility was overcome through preflight testing. To say that the B-29 had elaborate wind tunnel testing, is putting it mildly. Almost 9000 hr of continuous testing of scaled down parts on B-17s as well as exhaustive structural tests were conducted.
It should be pointed out that in tests of the wings after they had been installed on the fuselage section, more than 300,000 lb of pressure had to be applied to these surfaces. A complicated system of hydraulic jacks was developed which saved a tremendous amount of labor and provided more accurate application of the loads. This type of static testing was formerly done by piling sacks of lead shot on the wings. Handling this amount of lead in itself would have been a major undertaking.
In the drop test of the complete airframe, the Army stipulated that the plane must withstand a free drop of 27". The plane, with weight inside to simulate full equipment, was raised 27" from the floor and then dropped. Two drops, one in a horizontal and one in an inclined position, were made successfully.
Exacting static tests, which demonstrated the fundamental soundness of design, made it unnecessary for any major section of the plane to be redesigned.
Describing Superfortress, Gen H H Arnold characterized as "superb" the combat records of previous heavy bombers, the Flying Fortress and the Consolidated B-24 Liberator, but added that the B-29 "is as far ahead of those two aircraft as they are out in front of prewar bombers."
Army officials reveal that Superfortress' speed is more than 300 mph, that it has a ceiling of well above 30,000' and that its range is "extremely long." More than 1 year ago, Gen H H Arnold warned the Axis that planes were coming "which could fly the Atlantic, drop their bombs and return without stopping."
Significant is the fact that the plane will operate without an escort of fighters, for its range is far beyond that of fighter planes.
Two of its operating secrets are special electronic instruments enabling it to fly higher and more accurately than other aircraft. These are an electronic turbosupercharger regulator and an electronic automatic pilot, both produced by Minneapolis-Honeywell Regulator Co.
The latter device is capable of directing the plane on a given course with much greater accuracy than is possible with human piloting, and can make more than 330 flight corrections per min, even in violent cross-currents.
Initial taxi tests of the first XB-29 were made on September 9, 1942. Tests were continued through September 15, after which 5 days were spent in a complete check on the power plant and other equipment.
The maiden test flight was made on September 21, with the late Edmund T Allen, Boeing director of flight and aerodynamics, at the controls. Wellwood E Beall, vice president in charge of engineering, telegraphed simply but enthusiastically in his progress report to Washington, DC, that night:
"Eddie Allen reports that we have an excellent airplane." In that, the Nips concur.
This article was originally published as a special tip-in insert in the July, 1944, issue of Industrial Aviation magazine, vol 1, no 2, pp 30A-30D.
The original article includes 5 photos, a 3-view drawing and a data table.
Photos are not credited.